Upsert the attribute value for an end user entity attribute source

curl --request POST \
  --url https://app.herondata.io/api/end_users/{end_user_heron_id}/entities/{entity_heron_id} \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --data '{
  "attribute_heron_id": "<string>",
  "source_heron_id": "<string>",
  "value": "<any>"
}'

{
  "heron_id": "<string>",
  "value": "<any>"
}